The present invention relates to an inorganically insulated heater. More particularly, it relates to an inorganically insulated heater improved in the inorganic insulating layer thereof, a process for production thereof, and the use thereof.
In cathode ray tubes and air flow sensors, there have been used inorganically insulated heaters provided with an insulating layer formed of a porous layer of an inorganic substance.
In particular, the cathode heating heater of a cathode ray tube generally comprises as shown in FIG. 1 a metallic wire coil 1, an insulating layer 2 and a dark layer 5, the metallic wire coil 1 being in the form of a double coil twisted toward the return bend end 1a.
The insulating layer 2 of said heater is formed of inorganic insulating particles comprising alumina (Al.sub.2 O.sub.3) and the like as the main component. It is formed in close contact with the metallic wire surface.
The heater heats a cathode sleeve 3 formed cylindrically on the outside of the insulating layer 3, thereby heating a cathode pellet 4 attached to the end of the sleeve and making it emit thermoelectrons. The insulating layer 2 electrically insulates the cathode sleeve 3 from the metallic wire coil 1 [Japanese Patent Application Kokai (Laid-open) No. 57-95,035).
The dark layer 5 provided on the insulating layer 2 acts to enhance the heating efficiency [Japanese Patent Application Kokai (Laid-open) No. 59-132,537].
According to an experiment conducted by the present inventors it has been revealed that prior art cathode heating heaters give rise to imperfect insulation in a short period of time when the cathode pellet 4 is heated and operated at about 1100.degree. C. or above.
The main reasons for this are as follows. As shown schematically in FIG. 2, during the firing of the insulating layer 2, voids 10 and cracks 9 that can reach the surface of the insulating layer develop in the insulating part 8 present between adjacent metallic wires of the metallic wire coil (whereas they do not develop in the insulating part 7 present on the metallic wire coil). Consequently, the strength of the insulating layer is lowered, and troubles are apt to occur owing to (1) breakage of the insulating part 8 present between metallic wires due to the thermal shock caused by on-off of electricity through the metallic wire coil, (2) short-circuit between adjacent metallic wires and burnout thereof due to the breakage of the insulating part 8, and (3) dielectric breakdown due to the presence of voids 10 developed in the insulating layer [caused by voltage (about 300 V) applied between the metallic wire coil and the cathode sleeve].
As the means for solving such problems, it has been proposed to mix fibrous or whisker-formed high melting point inorganic insulating material with the inorganic insulating particles thereby increasing the strength of the insulating layer and prevent the development of said cracks [Japanese Patent Application Kokoku (Post-Exam. Publn.) No. 44-1,775] or, conversely, to increase the porosity of the insulating layer thereby hindering the extension of the cracks [Japanese Patent Application Kokai (Laid-open) No. 60-221,925].
Further, methods have been proposed which comprise forming the metallic wire coil and the insulating layer not in a closely contacted state but with a clearance provided therebetween, thereby hindering the development of cracks due to thermal strain or difference in thermal expansion [Japanese Patent Application Kokai (Laid-open) Nos. 61-121,232 and 61-142,625].
It has been found that although the above-mentioned means for preventing the development or extension of cracks are all effective for heaters operated at relatively low temperatures (about 1,100.degree. C. or below), they give only a short duration of life for heaters of the impregnation cathode heating system.
Insulating layers of the prior art have the following drawbacks.
(1) As shown in FIG. 2, it is difficult to prevent voids 10 or portions wherein the packing rate of the insulating particles is low (that is, non-uniform portions) from being formed between adjacent wires of metallic wire coil of the heater, so that the insulating layer is of low strength and is apt to undergo dielectric breakdown.
(2) Sintering of the inorganic insulating particles with each other proceeds during operation of the heater, causing contraction of the insulating layer, which results in development and progress of cracks, leading to dielectric breakdown in a short period of time.
(3) In the case of air flow sensors or such, though the working temperature is relatively low (about 200.degree. C.), they are subjected to strong vibration because they are mounted on automobiles or the like, and hence the insulating layer is apt to develop cracks.
The cathode heating heater of the cathode ray tube of the prior art is generally prepared as follows. A primary coil is formed by winding W wire or Re-containing W wire as the metallic wire for the metallic wire coil. The primary coil is then wound in a specified dimension round a core of molybdenum (Mo) to form a double coil. Then Al.sub.2 O.sub.3 particles are electrodeposition-coated thereon by means of electrophoresis and the like, and fired at 1600.degree.-1700.degree. C. to form an insulating layer composed of a porous layer of inorganic substance.
Then, according to intended purposes, either a dark layer comprising, for example, Al.sub.2 O.sub.3 particles and tungsten (W) particles is attached onto said insulating layer and then fired, or a dark layer is formed on the unfired insulating layer and then the insulating layer and the dark layer are fired at the same time.
After firing, the Mo core is removed by dissolution with an acid to leave a space 6 as shown in FIG. 2, and the remaining system is washed with water and dried to give the intended heater.
When an insulating layer is formed by electrodeposition on the double coil-formed metallic wire as shown in FIG. 1, the inorganic insulating particles are adhered onto the metallic wire by electrophoresis through a suspension (i.e., liquid containing particles of Al.sub.2 O.sub.3 etc. dispersed and suspended therein).
The driving force in said adhesion is attributed to a hydroxide gel formed by conversion of electrolytes, such as nitrates, dissolved in the suspension caused by electrolysis. However, although such gels are readily formed on the surface of metallic wire they are rather difficultly formed between the metal wires, so that voids are apt to develop in such places (Arato: Collected preliminary papers for 1987--spring meeting of Japan Inst. of Metals, p. 373).
This situation will be explained with reference to FIG. 2. Onto the insulating part 7 on the coil are adhered relatively small particles in the suspension relatively densely, while onto the insulating part 8 between adjacent metallic wires are adhered non-uniformly relatively large particles in the suspension.
Consequently, the insulating layer contracts between the metallic wire coils in the course of firing of the layer, resulting in development of cracks 9 or voids 10 [see FIG. 5 (b)].
Further, it has been revealed that, in the prior art heaters, contraction of the insulating layer caused by the progress of sintering of the layer which takes place during the operation of the heater, thermal shocks caused by thermo cycles, or repeated expansion and contraction of the metallic wire coil cause, in particular, breakage of the insulating part 8 of low strength present between metallic wires; and resultantly contact between metallic wires or metallic wire coils, breaking of wire of the heater, and dielectric breakdown of the insulating layer are apt to take place.